The invention is concerned with a power-split gear, consisting of an adjustable hydrostatic unit, of a five-shaft summing gear with a first, a second and a third output shaft, and of a range gear, the range shifts being carried out by the shifting of clutches.
A generic power-split gear with a five-shaft summing gear is known, for example, from FIG. 9 of DE 41 06 746 A1. Two of the three output shafts are in each case connectable to a clutch shaft via a clutch. This two clutches, and two others, between rotating parts require a supply of pressure medium via a shaft which, for this purpose, is a hollow shaft. Even six clutches are necessary altogether, thus not only increasing the design outlay, but also making the control more complicated. Moreover, the clutch shaft, in the form necessitated by its function, is unsuitable for cost-effective series production, and the ringwheel of the summing gear, said ringwheel being connected to the first clutch, is guided only by the planet wheels, this being insufficient and leading to the increased wear or poor grip of the clutch.
Another disadvantage is that the pull or rotational speed necessary for specific applications (for example, on tractors) cannot be achieved, and that the shifting time in the shuttle mode (alternately forward and backward) is too long because of the mass inertias in the range gear and because of the number of clutches involved.
It is an object of the present invention to provide a power-split gear which does not have these disadvantages and is to be capable of being produced and controlled more easily.
The foregoing object is achieved, according to the invention, wherein the range gear consists of a first planetary stage contiguous with the summing gear and of a second planetary stage, the first planetary stage has a sunwheel, a ringwheel and a web with first planet wheels, the sunwheel being drive-connected directly to the first output shaft and the ringwheel being connectable to the casing by means of a first clutch, the second planetary stage has a sunwheel, a ringwheel and a web with second planet wheels, the sunwheel being drive-connected directly to the second output shaft and the ringwheel being connectable to the casing by means of a second clutch, the third output shaft is connectable to a power take-off shaft by means of a third clutch, the web is common to the two planetary stages and is permanently connected fixedly to the power take-off shaft, and the second output shaft is connectable to the power take-off shaft via a fourth clutch.
The clutches for the two of the three output shafts of the summing gear which require a supply of pressure medium by means of the shaft are thus replaced by two clutches located downstream in the force flux, together with the spatially fixed casing. There are no longer any clutches between the summing gear and the range gear. As a result, the entire design is simplified substantially, there are fewer clutches and fewer rotary inlets for pressure medium, and there are no longer an uncentered ringwheel or parts unsuitable for series production. Due to the smaller number of clutches, the control behavior is also better and the changed arrangement allows more closely adapted range limits and therefore also more closer adaptation to extreme operating conditions.
In a particularly advantageous embodiment, the two planetary stages have identical static transmission ratios. This not only makes it possible to design a large number of parts as identical parts, but also assists in obtaining a better position of the synchronous rotational speeds for the range shifts.
The solution according to the invention is suitable for a wide variety of embodiments of the five-shaft summing gear. Various arrangements of suitable multiple planetary gears are described, for example, in DE 41 06 746. In a particularly space-saving embodiment of flexible design, the five-shaft summing gear has a web with first and second planet wheels meshing in each case with one another and also sunwheels and ringwheels meshing in each case with the first or second planet wheels, the ringwheel which meshes with the second planet wheels being connected to the first output shaft, the sunwheel which meshes with the second planet wheels being connected to the second output shaft and the web being connected to the third output shaft. A particularly simple tie-on of the sunwheels of the two planetary stages is also achieved thereby.
In an embodiment particularly suitable for the gear to be used in tractors, the power take-off shaft leads to a reversing gear. In this way, the reversing gear is connected downstream, and it may possess its own casing or share the casing with the power-split gear. In this case, the number of reverse speeds is identical to that of the forward speeds without any additional outlay in terms of construction. This affords considerable structural simplification and great advantages in the shuttle mode, particularly when the reversing gear is a planetary gear.
A particularly inexpensive and robust design of the reversing gear is achieved if it has an input sunwheel, an output sunwheel and a web with first and second planet wheels meshing with one another, the first planet wheels meshing with the input sunwheel and the second planet wheels meshing with the output sunwheel, and the web being connectable selectively to the input sunwheel via a forward clutch or to the casing via a reverse clutch. In contrast to the known planetary reversing gears, in which a ringwheel which is costly to produce is present and has to be braked, in this design it is the web that is braked; there is no ringwheel present. The planetary rotational speed is substantially lower as a result, this being beneficial to the useful life.
All the clutches used in the power-split gear according to the invention may, in principle, be of any desired type, in particular even friction clutches. However, positive clutches can be used to particular advantage in terms of control and space saving.